A prevalent process for the manufacturing of all-ceramic dental restorations in particular is the so-called press ceramics, in which dental ceramic material is converted to a viscous state under the impact of pressure and heat and is pressed in a form that corresponds to the dental prosthesis. The technique and a press ceramic furnace suitable for this technique are described for example in EP 0 231 773 A1 and has become known as Empress® process (Ivoclar, Schaan, Co., LI). The process is suitable for the manufacturing of (partial) crowns, inlays, onlays, veneers or bridges, and also for the manufacturing of frames for the so-called veneer ceramics technique. In this technique, metal frames are as a rule veneered with ceramics such that a tooth-colored dental prosthesis is finally formed. Correspondingly, all-ceramic frames are formed by means of the press ceramics. The advantage of these all-ceramic frames is that no dark metal can shine through from below the veneer.
The prevalent press ceramic materials attain breaking strengths of 220 MPa [Manfred Kern, Arbeitsgemeinschaft Keramik Prof. Dr. Lothar Pröbster (Wiesbaden-Tübingen) http://www.ag-keramik.de.news11whoiswho.htm]. The materials of the second generation (Empress® 2) are lithium disilicate glass ceramics and attain higher breaking strengths of 350-400 MPa (DE0019750794A1, DE0019647739C2, EP1149058A2). This is explained by the fact that the structure of the lithium disilicate crystals corresponds to the “Mikado” principle. Thus microcracks are prevented. A crack has to either overcome many boundary layers or extend around the crystals so that the crack energy is dissipated.
In dental ceramics, the term breaking strength or breaking boundary means the boundary in pressure load, tensile loading, bending load or torsion load up to which a material can be loaded without breaking. The collapse load is the force that is exerted up to the point of breakage. (Hoffmann-Axthelm, Lexikon der Zahnmedizin [Lexicon of Dentistry], 1983).
The bending strength of Empress® press ceramics equals 110 to 120 MPa (Volker Kluthe, Dissertation, http://darwin.inf.fu-berlin.de/2003/146/literatur.pdf). The fracture toughness lies approximately at 1.2. There is a need for materials that exceed at least a part of the said values if they are processed using the press ceramics process. There have been experiments already (DE 198 52 516 A1) to make available leucitic glass-ceramics for veneer frames that particularly have advantageous thermal expansion coefficients concerning the compatibility to veneer materials. DE 198 52 516 A1 does not go into the details of the mechanical properties.
The task is to make available a material that can be pressed at <1200° C. and has very good values in terms of breaking strength and fracture toughness.